The present invention relates generally to magnetic data storage devices. In particular, this invention relates to electrostatic actuators for fly height control.
Fly height between the read/write head and the magnetic disc surface in data storage devices has become smaller as the density of data stored increases. Increased density results in narrower data tracks and smaller data bits. These smaller dimensions require the read/write head to be closer to the disc surface to accurately interact with the disc. As the fly height decreases, it becomes increasingly difficult to prevent damage to the read/write head and disc surface. Undesired mechanical contact and electrical interaction between the read/write head and the disc surface can cause damage to those components or reduce performance.
Prior attempts to control fly height include the use of electrostatic actuators. Electrostatic actuators make use of two electrodes, between which an electric field is maintained to control the distance between the electrodes. One electrode is formed by or deposited on the air bearing slider that carries the read/write head. The other electrode is formed by the magnetic disc media. Electrostatic actuators control fly height by a generated attractive force between the slider and the disc surface that increases as the fly height decreases. If the attractive force increases too much, it can result in collisions and near collisions between the read/write head and the disc surface. The problem of collision and near collision is compounded by slight variations in the disc topography related to manufacturing limitations. Collision results in a discharge from the electrode due to tunneling current. Tunneling current is the undesired flow of electrons across the fly height gap. Collision may also result in physical damage to the disc surface due to rough contact with the electrode. Near collision results in field emission discharge between the slider electrode and the disc. Field emission discharge is the undesired loss of potential due to a large potential present across a small gap.
Electrostatic actuators must also maintain relatively fast response times. The fly height actuator receives voltage input from a fly height controller. The fly height controller generates a fly height control voltage directly proportional to a sensed fly height. A feedback loop is maintained between the fly height controller and the fly height actuator in order to maintain a desired fly height. To ensure the fly height is set to the proper height at the appropriate time by the fly height actuator, as called for by the fly height controller, the fly height electrode must have rapid response time.
In the case of sliders composed of conductive material, such as AlTiC, previous electrostatic actuators have used the slider surface as one of the electrodes necessary for the operation of the fly height control actuator, see published U.S. application Ser. No. 20020097517, Bonin et al. Improvements on this method have been made by using a separate metal electrode device insulated from the slider to serve as one electrode of the fly height control actuator, see published U.S. application Ser. No. 20030043497, Riddering et al. It is advantageous to use an isolated electrode instead of the slider surface because it becomes necessary to only supply voltage to the electrode instead of the whole slider to activate the fly height control actuator. Having the whole slider supplied with voltage creates the danger of short circuit and discharge of the electrode when contact between the slider and disc media occurs. Electrodes can also be positioned such that it is more likely the grounded slider contacts the disc rather than the electrode in order to avoid short circuit and discharge.